When you begin the mission to search for the Green Minions, you will have to sneak into a castle using your Minions. While you are in there, you will discover a few pressure plates that release gnomes as a distraction to the enemies. However, as long as there are still enemies in the castle the gnomes will keep spawning. This allows you to not only get the 1,000 kills of a gnome quickly, you will also be collecting crystals, gold, and Lifeforce orbs while doing so.